<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<style>
    * {
        margin: 0;
        padding: 0;
    }
    .box{ background:skyblue; height: 200px}
    .kid{ width: 100px;height: 100px; background: yellow; margin-top: 20px}
</style>

<body>
<div class="box">
    <div class="kid">子元素1</div>
</div>
</body>
</body>
</html>